Researchers use in situ NMR spectroscopy to provide insight into silicon expansion in Li-ion batteries

4 February 2014

A major barrier to the use of high energy capacity silicon in a lithium-ion battery is the volumetric expansion of silicon under lithiation and delithiation, which results in electrode degradation and capacity fade. (Earlier post.) Although researchers have made progress in designing materials to mitigate this effect, a lack of understanding about what is happening inside the batteries and what governs the reactions continues to retard commercialization. Now, researchers at the University of Cambridge report developing a new in situ method to probe batteries with silicon electrode and determining what causes the expansion to take place. A paper on their work is in the journal Nature Communications.

A 2012 image from Argonne National Laboratory (not directly related to this new Cambridge work) illustrates the volumetric expansion problem with Si anodes in Li-ion batteries. Silicon (shown in grey) is capable of holding 10 times as many lithium ions (shown in pink) as currently-used anodes. This image shows the large volume expansion that silicon undergoes as it soaks up the lithium. Artist: Rees Rankin (CNM) Researchers: Maria Chan (CNM), Chris Wolverton (Northwestern University), Jeff Greeley (CNM) Click to enlarge.

Using nanoscale wires made of silicon and NMR techniques, the researchers developed a robust model system able to accommodate the expansion of the silicon over multiple cycles, and integrated it with short-range probing techniques that reveal what is happening inside the battery at the atomic level. The team found that the reactions proceed with interactions of various sizes of silicon networks and clusters, energetics of which partly govern the path of the reaction.

The method allows the (de)alloying reactions of the amorphous silicides to be followed in the 2nd cycle and beyond. In combination with density-functional theory calculations, the results provide insight into the amorphous and amorphous-to-crystalline lithium–silicide transformations, particularly those at low voltages, which are highly relevant to practical cycling strategies.

—Ogata et al.

Using these combined techniques, the researchers were able to develop a map of how silicon transforms when it is put into contact with lithium in a battery. The insights opened up by the technology will boost further developments of silicon batteries, as it will be easier for engineers to control their properties.

Using this technique will help make battery design much more systematic, and less trial and error. The nanowire-based batteries coupled with the NMR system enabled us to follow the reaction kinetics over multiple cycles with various cycling strategies. Importantly, the insights achieved by the new technology are relevant to current state-of-the-art silicon-carbon composite anodes and will lead to further development of the anodes.

—Dr. Ken Ogata, lead author

This versatile nanowire-based technology can be applied to other battery system such as tin and germanium-based lithium-ion batteries and sodium-ion batteries, and studies are currently on going with the NMR spectroscopy under a wide variety of electrochemical regimes.